Author Topic: BFR with expendable upper stage  (Read 14108 times)

Offline Michael Bloxham

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BFR with expendable upper stage
« on: 02/13/2018 02:41 AM »
Hey guys,

I'm wondering what the LEO and TMI performance of the BFR would be with an expendable version of the BFS acting as an upper stage? Assume no delta wings, no TPS, and deleted landing engines (so just 4x vacuum Raptors).

The scenario I'm thinking about is if reusable BFS is put in the too-hard basket, or is found suitable for LEO ops only. Or the economics of large-scale fuel manufacturing on Mars doesn't work out (at least for initial missions!)...

The idea of an expendable upper could be quite compelling given the extra TMI performance out of it. With a hammer-head PLF you could fit some very large diameter Mars landers in there, thereby keeping lander density low, easing EDL requirements.

I also found a Reddit thread talking about Falcon Heavy TMI performance with expendable rather than reusable central booster. It implied costs per kg were lower for the expendable case, because of the extra performance.

Cheers,

- Mike

Online Robotbeat

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Re: BFR with expendable upper stage
« Reply #1 on: 02/13/2018 02:53 AM »
Boo. Theyíll keep trying reusable upper stage until they go bankrupt.
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #2 on: 02/13/2018 02:57 AM »
Iím sure theyíll have the reusable upper stage eventually, for LEO and maybe even GEO payloads. What Iím not sure about is the economics for the whole Mars thing.

Offline Prettz

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Re: BFR with expendable upper stage
« Reply #3 on: 02/13/2018 03:00 AM »
If it's expendable they can't afford to launch it in the first place, so what's the point?

Online Robotbeat

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Re: BFR with expendable upper stage
« Reply #4 on: 02/13/2018 03:06 AM »
Iím sure theyíll have the reusable upper stage eventually, for LEO and maybe even GEO payloads. What Iím not sure about is the economics for the whole Mars thing.
You're confused because you're using direct-launch performance numbers to guide your intution. You basically can't actually direct-launch anything to Mars with a reusable two stage rocket. But that's irrelevant because you refuel the reusable upper stage in orbit. That is way more efficient anyway.

Don't ignore refueling. Refueling changes everything.
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #5 on: 02/13/2018 03:17 AM »
Iím not a fan of orbital-refuelling when it comes to Mars missions. With Mars EDL you want a low-density lander, so low surface mass and wide aeroshell diameters. Getting through the bottleneck of EDL is hard. With orbital refuelling, you just end up increasing the density of your payload. It doesnít help you at all. Iím not sure if SpaceX will achieve their EDL goals with BFS.

In a lot of Mars mission studies, the upper stage of the launcher also serves as the TMI stage, so I donít see why I canít assume the same with BFR.

Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #6 on: 02/13/2018 03:20 AM »
If it's expendable they can't afford to launch it in the first place, so what's the point?

If you assume very expensive payloads (as in a Mars mission), then expending the upper stage could make sense, given the increased performance. Youíre still reusing the whole first stage.

Offline RonM

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Re: BFR with expendable upper stage
« Reply #7 on: 02/13/2018 03:25 AM »
Iím not a fan of orbital-refuelling when it comes to Mars missions.

That's the key step in the BFR/BFS architecture. Obviously, Elon Musk likes orbital refueling.

Online Robotbeat

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Re: BFR with expendable upper stage
« Reply #8 on: 02/13/2018 03:31 AM »
I’m not a fan of orbital-refuelling when it comes to Mars missions. With Mars EDL you want a low-density lander, so low surface mass and wide aeroshell diameters. Getting through the bottleneck of EDL is hard. With orbital refuelling, you just end up increasing the density of your payload. ...
I can't see how you arrive at that conclusion. Nearly all the propellant you're refuelling with orbital refueling is burned doing TMI, so you end up with a low density "payload" (i.e. the entire BFS) with a high effective aeroshell area.

It seems you're not familiar with how BFS is planned to work. Well, you're in for a treat! Here's the 2016 video. BFR/BFS is essentially the same concept, just with a slightly smaller vehicle. Please watch the whole thing.

« Last Edit: 02/13/2018 03:32 AM by Robotbeat »
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Online Robotbeat

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Re: BFR with expendable upper stage
« Reply #9 on: 02/13/2018 03:33 AM »
If you can't make the reusable upper stage work, then you don't have a Mars lander or aeroshell, either. They're the same vehicle. This is a key innovation in the BFR/ITS architecture. Please watch the above video.
« Last Edit: 02/13/2018 03:34 AM by Robotbeat »
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #10 on: 02/13/2018 03:40 AM »
But then you end up with this huge expensive BFS on Mars surface, which you need to refuel to send back. The whole idea of reusing the BFS for Mars relies on huge refuelling infrastructure on Mars. If this ends up being uneconomic, then the benefit of BFS for Mars goes away (still useful for LEO tho). Also, even with successful reuse, youíre still looking at the expense of setting up the fuel manufacturing on Mars in the first place. Youíre also taking the productive asset that is BFS and tying it up with incredibly infrequent Mars missions (one launch every 2.5 years). It might be better to use that asset for LEO work, where it could be flown hundreds of times per year. Thereís always a trade off. Iím suggesting it might make sense to go the expendable route for missions beyond LEO.

Offline AC in NC

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Re: BFR with expendable upper stage
« Reply #11 on: 02/13/2018 03:42 AM »
Pretty sure they know how, by this point, to have run the numbers and decided you're wrong.

Online Robotbeat

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Re: BFR with expendable upper stage
« Reply #12 on: 02/13/2018 03:46 AM »
Having a big refueling station on Mars isn’t that hard if you can land huge payloads on Mars cheaply. And BFS isn’t very expensive, on the order of $120m, still much cheaper than any comparable expendable lander.

The dream of a Mars City disappears without upper stage reuse and without reusable Mars landers. Full stop.
« Last Edit: 02/13/2018 03:48 AM by Robotbeat »
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #13 on: 02/13/2018 03:48 AM »
Pretty sure they know how, by this point, to have run the numbers and decided you're wrong.

Iím sure theyíre still working on their numbers. We havenít even seen their proposal for setting up the fuel manufacturing on Mars surface yet.
« Last Edit: 02/13/2018 03:50 AM by Michael Bloxham »

Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #14 on: 02/13/2018 03:55 AM »
The dream of a Mars City disappears without upper stage reuse and without reusable Mars landers. Full stop.

I disagree. You could build a nice Mars city using expendable landers. Itís down-mass (and volume) that you want. Which fits quite nicely with the idea of using large low-density aeroshells for surface payloads. BFS is volume restricted.

Online Robotbeat

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Re: BFR with expendable upper stage
« Reply #15 on: 02/13/2018 03:59 AM »
The dream of a Mars City disappears without upper stage reuse and without reusable Mars landers. Full stop.

I disagree. You could build a nice Mars city using expendable landers. It’s down-mass (and volume) that you want. Which fits quite nicely with the idea of using large low-density aeroshells for surface payloads. BFS is volume restricted.
Nope. Can’t afford the downmass with expendable. Cannot and will not happen.

Luckily, there’s no magical law of physics preventing reusable upper stages and landers from happening. We already know it’s possible.
« Last Edit: 02/13/2018 03:59 AM by Robotbeat »
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #16 on: 02/13/2018 04:01 AM »
It comes down to the cost of setting up the fuel manufacturing and launch ops infrastructure.

Online Robotbeat

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Re: BFR with expendable upper stage
« Reply #17 on: 02/13/2018 04:07 AM »
It comes down to the cost of setting up the fuel manufacturing and launch ops infrastructure.
...all of which is cheaper if you have the capability to land large payloads.

Oh, and if you don’t like that BFS is only used once per 26 months (although in several different roles, so effectively reused several times within 26 months), then there are ways to launch several payloads to Mars ballistically, then capture them at Mars and bring them down with a refueled BFS. So the one use per synod is not at all a hard and fast rule.
« Last Edit: 02/13/2018 04:07 AM by Robotbeat »
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #18 on: 02/13/2018 04:09 AM »
If you have to do 20+ flights of BFR/BFS to LEO to set up the infrastructure to return one BFS from Mars surface... Maybe it could work out cheaper to just to do one flight of BFR and an expendable BFS derived upper stage with a big dumb lander. The big dumb lander could help you set up the infrastructure more cheaply. Full reusability could come later, if itís desirable at all by that stage. You could just get straight into base-building with a big dumb lander.

Online Robotbeat

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Re: BFR with expendable upper stage
« Reply #19 on: 02/13/2018 04:11 AM »
Without upper stage reuse, launch costs will never approach the low levels needed for mass settlement. With partial reuse we can get down to $1000/kg. We might need to get nearly $10/kg (consistent with spaceXís numbers for ITS/BFR) to really make mass settlement feasible. Just not possible without upper stage reuse.
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